Liquid crystals are widely used for electronic displays. In these display systems, a liquid crystal cell is typically situated between a pair of polarizer and analyzers. An incident light polarized by the polarizer passes through a liquid crystal cell and is affected by the molecular orientation of the liquid crystal, which can be altered by the application of a voltage across the cell. The altered light goes into the analyzer. By employing this principle, the transmission of light from an external source, including ambient light, can be controlled. The energy required to achieve this control is generally much less than required for the luminescent materials used in other display types such as cathode ray tubes (CRT). Accordingly, liquid crystal technology is used for a number of electronic imaging devices, including but not limited to digital watches, calculators, portable computers, electronic games for which light-weight, low-power consumption and long-operating life are important features.
Contrast, color reproduction, and stable gray scale intensities are important quality attributes for electronic displays, which employ liquid crystal technology. The primary factor limiting the contrast of a liquid crystal display (LCD) is the propensity for light to “leak” through liquid crystal elements or cells, which are in the dark or “black” pixel state. Furthermore, the leakage and hence contrast of a liquid crystal display are also dependent on the direction from which the display screen is viewed. Typically the optimum contrast is observed only within a narrow viewing angle range centered about the normal incidence to the display and falls off rapidly as the viewing direction deviates from the display normal. In color displays, the leakage problem not only degrades the contrast but also causes color or hue shifts with an associated degradation of color reproduction.
LCDs are quickly replacing CRTs as monitors for desktop computers and other office or house hold appliances. It is also expected that the number of LCD television monitors with a larger screen size will sharply increase in the near future. However, unless problems of viewing angle dependence such as coloration, degradation in contrast, and an inversion of brightness are solved, LCD's application as a replacement of the traditional CRT will be limited.
A Vertically-Aligned liquid crystal display (VA-LCD) offers an extremely high contrast ratio for normal incident light. FIG. 2A and FIG. 2B are the schematics of VA liquid crystal cell in OFF 201 and ON 203 states. In its OFF state, the liquid crystal optic axis 205 is almost perpendicular to the substrate 207, FIG. 2A. With an applied voltage, the optic axis 205 is tilted away from the cell normal, FIG. 2B. In the OFF state, light does not see the birefringence in the normal direction 209, giving the dark state that is close to that of orthogonally crossed polarizers. However, obliquely propagated light 211 picks up phase retardation giving light leakage. This results in a poor contrast ratio in some viewing angle range.
A bend aligned nematic liquid crystal display, also referred as an Optically Compensated Bend Liquid Crystal Display (OCB-LCD) uses a nematic liquid crystal cell based on the symmetric bend state. In its actual operation, the brightness of the display using the bend aligned nematic liquid crystal cell is controlled by an applied voltage or field that leads to a different degree in the bend orientation within the cell as shown in FIG. 3A (OFF) 301 and FIG. 3B (ON) 303. In both states, the liquid crystal optic axis 305 takes symmetric bend state around the cell middle plane 307. In the On state, the optic axis becomes substantially perpendicular to the cell plane except near the cell substrates 309. OCB mode offers faster response speed that is suitable to the liquid crystal display television (LCD-TV) application. It also has advantages in viewing angle characteristic (VAC) over conventional displays, such as Twisted Nematic liquid crystal display (TN-LCD)
The above-mentioned two modes, due to their superiority over the conventional TN-LCD, are expected to dominate the high-end application such as LCD-TV. However, practical applications of both OCB and VA-LCDs require optical compensating means to optimize the VAC. In both modes, due to the birefringence of liquid crystal and crossed polarizer, VAC suffers deterioration in contrast when the displays are viewed from oblique angles. Use of biaxial films have been suggested to compensate the OCB (U.S. Pat. No. 6,108,058) and VA (JP1999-95208) LCDs. In both modes, liquid crystals align sufficiently perpendicular to the plane of the cell in ON(OCB) or OFF (VA) states. This state gives positive Rth, thus the compensation films have to have sufficiently large negative Rth for satisfactory optical compensation. The need for a biaxial film with a large Rth is also common for Super Twisted Nematic Liquid Crystal Display (STN-LCD).
Several methods of manufacturing biaxial films with sufficient negative value of Rth suitable for compensating LCD modes such as OCB, VA and STN have been suggested.
US 2001/0026338 discloses a use of retardation increasing agent in combination with triacetylcellulose (TAC). The retardation-increasing agent is chosen from aromatic compounds having at least two benzene rings. By stretching agent doped TAC, one can generate both Rth and Rin. The problems with this method is that the amount of the doping of the agent. To generate the desired effects of increasing Rth and Rin the necessary amount of agent is high enough to cause coloration. With this method, it is difficult to control the values of Rth and Rin independently.
Sasaki et al. proposes (US2003/0086033) the use of cholesteric liquid crystal disposed on the positively birefringent thermoplastic substrate. The pitch of the cholesteric liquid crystal (CHLC) is shorter than the wavelength of the visible light, thus properly aligned CHLC exhibits form birefringence giving negative Rth. Rin is controlled by adjusting the stretching amount of the thermoplastic substrate. The method enables one to adjust Rth and Rin separately. However, the use of short pitch CHLC not only makes the manufacturing cost high but also complicates the processing due to the alignment procedure.
JP2002-210766 discloses the use of propionyl or butyryl substituted TAC. They show higher birefringence than ordinary TAC. Thus, by biaxially stretching the substituted TAC film, one generates Rin and Rth. The method does not require any additional coating or layer but it suffers a difficulty of independent control of Rin and Rth.
Thus, it is a problem to be solved to provide a multilayer optical compensator with independently controlled Rth and Rin that can be readily manufactured.